1. Field of the Invention
The present invention relates to an image recording apparatus for recording images, and more particularly to an image recording apparatus capable of recording halftone-processed images.
2. Related Background Art
As an image recording apparatus for recording images, there are known ink jet recording apparatus, thermal transfer recording apparatus and the like. The halftone representation method used for these recording apparatus is sub-divided mainly into a dot density control method wherein the number of fixed size dots per unit area is controlled, and a dot diameter control method wherein the size of a dot is controlled.
Since the dot diameter control method has some limitation depending upon its recording method, the dot density control method is generally used. One typical binarization method for halftone representation used with the dot density control method is an ordered dithering method. This method, however, is associated with the problem that the number of grey levels depends upon the matrix size. Specifically, if the matrix size is increase large in order to make the number of grey levels, the size of one record pixel made from the matrix, also becomes large, thereby degrading deteriorate the image resolution.
Another typical binarization method is a conditional determination type dithering method such as an error diffusion method. In this method, the threshold value is changed on the basis of the peripheral pixels, of an input pixel or a dither signal is added, whereas in the above-described ordered dithering method, an unconditional determination type dithering is used, wherein a threshold value independent of the input pixel is used in binarization.
The conditional determination type dithering method, typically the error diffusion method, affords good compatibility between tonal rendition and resolution. In addition, if the original image is a print image, a moire and pattern rarely occurs to the recorded image.
The error diffusion method, however, has the problem that granular noise becomes conspicuous at the light portion of a processed image. Granular noises become more conspicuous for the case of a recording apparatus having a low (e.g., lower than 300 dpi) recording density. The assignee of the present invention has filed U.S. patent application Ser. No. 289,017 (application date Dec. 23, 1988), now U.S. Pat. No. 4,975,786, which discloses a method of reducing granular noise caused by the processing of the error diffusion method by not recording a dot of a low density data.
In order to make granular noise visually not conspicuous, there is known an ink jet recording apparatus of the type wherein are mounted two recording heads, one for jetting out light ink and the other for dark ink, whereby the light to halftone portion of an image is formed with recording dots of light ink, and the halftone to dark portion is formed with dark ink. In this case, however, there arise the problems that the image signal processing algorithm becomes complicated, a pseudo contour is likely to be generated because of discontinuity of smooth change of toner rendition at switching points between light and an dark inks, and unnatural image is produced because of change in granular noise and color tone at the switching points.
The above problems will become more serious if the reflection density of recording dots of light and dark inks is maintained within a narrow tolerance.
There is also proposed an ink jet recording apparatus of the type that there are mounted two recording heads, one having small diameter nozzles and the other large diameter nozzles, wherein the light to halftone portion of an image is recorded with small diameter dots, and the halftone to dark portion with large diameter dots, to thereby make granular noise not conspicuous. This ink jet recording apparatus is associated with the similar problem that a psuedo-contour is likely to be generated at the switching points between small and large dots.